1. Field of the Invention
The present invention relates to a molded structure comprising a thermoplastic synthetic resin which contains at least 10% by volume of open cells, a process for the production of a molded structure and the use of the molded structure.
The molded structure is an open-cell expanded plastic foam which, in contrast to an expanded plastic foam having closed cells, is capable of absorbing liquids.
2. Description of Related Art
Expanded plastic foams have long been known; in particular, expanded polystyrene foams have a wide range of uses. They are used, for example, as packaging shells and in the building sector, especially as insulating material, for example, as an underlay for wallpaper.
Expanded plastic foams are generally produced by extruding a thermoplastic synthetic resin together with a blowing agent, in particular a gaseous blowing agent, from an annular or slot die or coextrusion die. Shortly after leaving the extrusion die, the blowing agent finely distributed in the plastic expands so that very fine bubbles form in the expanded plastic molding. Smaller or larger bubbles are obtained depending on the process conditions with regard to the temperature and the extrusion rate. Accordingly, the expanded plastic foam thus produced contains densely packed, closed, gas-filled cells, the gas enclosed in the cells generally being air and/or residues of the blowing agent. The excellent insulating properties of the expanded plastic foams are essentially based on the densely packed, closed, gas-filled cells which to a certain extent form an "immobile" gas, in particular air, layer. Such an "immobile" air layer has poor thermal conductivity.
An expanded plastic foam in which the cells are not closed but open, i.e., have a through passage, shows completely different behavior. Such an expanded plastic foam can, for example, store liquids similarly to a sponge. In the case of such open-cell foams, the cells are connected to one another via the gas phase. In the extreme case, they comprise only cell webs.
The document WO 90/14159 describes an oil-absorbing material which comprises comminuted polyethylene foam, but this document gives no indication as to how an intact absorptive expanded plastic foam is to be obtained. By exerting the mechanical pressure on the expanded plastic foam, a major part of the previously closed cells of the polyethylene foam can be opened. However, the mechanical strength and the stability of the expanded plastic foam are substantially reduced since some of the cell walls are deformed by the mechanical pressure. Such a film may contain more than 50% of open cells.
EP-A-0 090 507 discloses shell-like packaging for fast food, which packaging comprises an open-cell expanded polymer layer and a closed-cell polymeric outer layer as a liquid barrier layer. For example, polystyrene is used as the polymer for both layers. The open-cell expanded plastic foam is formed in the same way as the extrusion of the closed-cell outer layer, with the single exception that an excess of nucleating agent, such as, for example, sodium bicarbonate and citric acid, is added in a weight ratio of from 0.8 to 1.2 and from 1.2 to 0.8, but the use of a blowing agent is dispensed with. Since no blowing agent is used, only a single mixing chamber and a single extrusion screw are required. In this known process, an open-cell expanded polystyrene layer having an average content of open cells of 50 to 90% by volume is obtained. However, this known process does not give a cell structure in which the individual cells are connected to one another; instead, only the cells in the surface of the expanded plastic foam are connected to the adjacent cells underneath, but a cell structure in which the cells are directly adjacent to one another and are separated from one another only by the cell walls is not obtained.
Japanese Patent No. 3-57348 discloses a moisture-absorbing packaging material comprising a molding which contains a homogeneous mixture of 50 to 85% by weight of ethylene polymer and 15 to 50% by weight of an adsorbent which does not swell on absorbing water.
Japanese Patent No. 54-31026 describes a process for the production of a polypropylene foam having an open-cell structure, in which homopolymers or copolymers of propylene are gelled under pressure and with heat, with the addition of chlorofluorocarbons as blowing agents or expanding agents and of a conventional nucleating agent. The extruded polypropylene foam has a uniform and fine cell structure possessing a mean cell diameter of 0.5 mm and a density of about 0.028 g/cm.sup.3. The absorptivity of the polypropylene foam for water is 3 to 7.6 times the foam density.